Social isolation increases cell proliferation in male and cell survival in female California mice (Peromyscus californicus)

Pair bonding and disruption impact lung transcriptome in monogamous Peromyscus californicus

Social interactions affect physiological and pathological processes, yet their direct impact in peripheral tissues remains elusive. Recently we showed that disruption of pair bonds in monogamous Peromyscus californicus promotes lung tumorigenesis, pointing to a direct effect of bonding status in the periphery (Naderi et al., 2021). Here we show that lung transcriptomes of tumor-free Peromyscus are altered in a manner that depends on pair bonding and superseding the impact of genetic relevance between siblings. Pathways affected involve response to hypoxia and heart development. These effects are consistent with the profile of the serum proteome of bonded and bond-disrupted Peromyscus and were extended to lung cancer cells cultured in vitro, with sera from animals that differ in bonding experiences. In this setting, the species' origin of serum (deer mouse vs FBS) is the most potent discriminator of RNA expression profiles, followed by bonding status. By analyzing the transcriptomes of lung cancer cells exposed to deer mouse sera, an expression signature was developed that discriminates cells according to the history of social interactions and possesses prognostic significance when applied to primary human lung cancers. The results suggest that present and past social experiences modulate the expression profile of peripheral tissues such as the lungs, in a manner that impacts physiological processes and may affect disease outcomes. Furthermore, they show that besides the direct effects of the hormones that regulate bonding behavior, physiological changes influencing oxygen metabolism may contribute to the adverse effects of bond disruption.

In the pursuit of new social neurons. Neurogenesis and social behavior in mice: A systematic review

Social behaviors have become more relevant to our understanding of the human nervous system because relationships with our peers may require and modulate adult neurogenesis. Here, we review the pieces of evidence we have to date for the divergence of social behaviors in mice by modulation of adult neurogenesis or if social behaviors and the social environment can drive a change in neurogenic processes. Social recognition and memory are deeply affected by antimitotic drugs and irradiation, while NSC transgenic mice may run with lower levels of social discrimination. Interestingly, social living conditions can create a big impact on neurogenesis. Social isolation and social defeat reduce the number of new neurons, while social dominance and enrichment of the social environment increase their number. These new “social neurons” trigger functional modifications with amazing transgenerational effects. All of these suggest that we are facing two bidirectional intertwined variables, and the great challenge now is to understand the cellular and genetic mechanisms that allow this relationship to be used therapeutically.

The Impact of Ethologically Relevant Stressors on Adult Mammalian Neurogenesis

Adult neurogenesis—the formation and functional integration of adult-generated neurons—remains a hot neuroscience topic. Decades of research have identified numerous endogenous (such as neurotransmitters and hormones) and exogenous (such as environmental enrichment and exercise) factors that regulate the various neurogenic stages. Stress, an exogenous factor, has received a lot of attention. Despite the large number of reviews discussing the impact of stress on adult neurogenesis, no systematic review on ethologically relevant stressors exists to date. The current review details the effects of conspecifically-induced psychosocial stress (specifically looking at the lack or disruption of social interactions and confrontation) as well as non-conspecifically-induced stress on mammalian adult neurogenesis. The underlying mechanisms, as well as the possible functional role of the altered neurogenesis level, are also discussed. The reviewed data suggest that ethologically relevant stressors reduce adult neurogenesis.

Social environment affects central distribution of estrogen receptor-α in Peromyscus californicus

Social environment has well-established effects on an animal's social behavior and associated neuroendocrine responses. The presence of estrogen receptor alpha (ERα) in limbic system brain regions is related to the expression of a variety of social, reproductive and aggressive behaviors. We hypothesized that alterations to the social environment, specifically social isolation, would cause changes in ERα throughout the limbic system. The number of ERα immunoreactive (ERα-ir) cells within specific limbic system brain regions was quantified in male and female California mice (Peromyscus californicus), isolated or same sex pair-housed for 4 or 24 days. Peromyscus californicus is a highly social rodent species (monogamous and bi-parental) and therefore, may be particularly sensitive to manipulations of its social environment. Isolated males had a significantly greater number of ERα-ir cells in the ventromedial nucleus of the hypothalamus (VMH) and similar patterns within the bed nucleus of the stria terminalis (BST) and medial preoptic area (MPOA). Males housed for 24 days had a significantly greater number of ERα-ir cells in the BST, VMH, MPOA when compared with males housed for 4 days. Females housed for 24 days had significantly greater ERα-ir in the dentate gyrus of the hippocampus (DG) when compared with females housed for 4 days. No differences were found in the medial amygdala (MeA). These data demonstrate that social environment has region and sex specific effects on ERα-ir cells in this species. These results add to the comparative evidence regarding ERα, demonstrating a consistent role for ERα in species specific responsiveness to changes in the social environment.

Neurobiology and consequences of social isolation stress in animal model-A comprehensive review

The brain is a vital organ, susceptible to alterations under genetic influences and environmental experiences. Social isolation (SI) acts as a stressor which results in alterations in reactivity to stress, social behavior, function of neurochemical and neuroendocrine system, physiological, anatomical and behavioral changes in both animal and humans. During early stages of life, acute or chronic SIS has been proposed to show signs and symptoms of psychiatric and neurological disorders such as anxiety, depression, schizophrenia, epilepsy and memory loss. Exposure to social isolation stress induces a variety of endocrinological changes including the activation of hypothalamic-pituitary-adrenal (HPA) axis, culminating in the release of glucocorticoids (GCs), release of catecholamines, activation of the sympatho-adrenomedullary system, release of Oxytocin and vasopressin. In several regions of the central nervous system (CNS), SIS alters the level of neurotransmitter such as dopamine, serotonin, gamma aminobutyric acid (GABA), glutamate, nitrergic system and adrenaline as well as leads to alteration in receptor sensitivity of N-methyl-D-aspartate (NMDA) and opioid system. A change in the function of oxidative and nitrosative stress (O&NS) mediated mitochondrial dysfunction, inflammatory factors, neurotrophins and neurotrophicfactors (NTFs), early growth response transcription factor genes (Egr) and C-Fos expression are also involved as a pathophysiological consequences of SIS which induce neurological and psychiatric disorders.

Separation increases passive stress-coping behaviors during forced swim and alters hippocampal dendritic morphology in California mice

Individuals within monogamous species form bonds that may buffer against the negative effects of stress on physiology and behavior. In some species, involuntary termination of the mother-offspring bond results in increased symptoms of negative affect in the mother, suggesting that the parent-offspring bond may be equally as important as the pair bond. To our knowledge, the extent to which affect in paternal rodents is altered by involuntary termination of the father-offspring bond is currently unknown. Here, we investigated to what extent separation and paternal experience alters passive stress-coping behaviors and dendritic morphology in hippocampal subfields of California mice (Peromyscus californicus). Irrespective of paternal experience, separated mice displayed shorter latencies to the first bout of immobility, longer durations of immobility, and more bouts of immobility than control (non-separated) mice. This effect of separation was exacerbated by paternal experience in some measures of behavioral despair—separation from offspring further decreased the latency to immobility and increased bouts of immobility. In the dentate gyrus, separation reduced dendritic spine density regardless of paternal experience. Increased spine density was observed on CA1 basal, but not apical, dendrites following paternal experience. Regardless of offspring presence, fatherhood was associated with reduced apical dendritic spine density in area CA3 of the hippocampus. Separation enhanced complexity of both basal and apical dendrites in CA1, while fatherhood reduced dendritic complexity in this region. Our data suggest that forced dissolution of the pair bond induces passive stress-coping behaviors and contributes to region-specific alterations in hippocampal structure in California mouse males.

Interaction Effect of Social Isolation and High Dose Corticosteroid on Neurogenesis and Emotional Behavior

Hypercortisolemia is one of the clinical features found in depressed patients. This clinical feature has been mimicked in animal studies via application of exogenous corticosterone (CORT). Previous studies suggested that CORT can induce behavioral disturbance in anxious-depressive like behavior, which is associated with suppressed neurogenesis. Hippocampal neurogenesis plays an important role in adult cognitive and behavioral regulation. Its suppression may thus lead to neuropsychiatric disorders. Similar to the effects of CORT on the animals’ depression-like behaviors and neurogenesis, social deprivation has been regarded as one factor that predicts poor prognosis in depression. Furthermore, social isolation is regarded as a stressor to social animals including experimental rodents. Hence, this study aims to examine if social isolation would induce further emotional or anxiety-like behavior disturbance and suppress neurogenesis in an experimental model that was repeatedly treated with CORT. Sprague-Dawley rats were used in this study to determine the effects of different housing conditions, either social isolated or group housing, in vehicle-treated control and CORT-treated animals. Forced swimming test (FST), open field test (OFT) and social interaction test (SIT) were used to assess depression-like, anxiety-like and social behaviors respectively. Immunohistochemistry was performed to quantify the number of proliferative cells and immature neurons in the hippocampus, while dendritic maturation of immature neurons was analyzed by Sholl analysis. Social isolation reduced latency to immobility in FST. Furthermore, social isolation could significantly reduce the ratio of doublecortin and bromodeoxyuridine (BrdU) positive cells of the neurogenesis assay under CORT-treated condition. The current findings suggested that the behavioral and neurological effect of social isolation is dependent on the condition of hypercortisolemia. Furthermore, social isolation may possibly augment the signs and symptoms of depressed patients with potential alteration in neurogenesis.

Contrasting effects of opposite- versus same-sex housing on hormones, behavior and neurogenesis in a eusocial mammal

Competitive interactions can have striking and enduring effects on behavior, but the mechanisms underlying this experience-induced plasticity are unclear, particularly in females. Naked mole-rat (NMR) colonies are characterized by the strictest social and reproductive hierarchy among mammals, and represent an ideal system for studies of social competition. In large matriarchal colonies, breeding is monopolized by one female and 1-3 males, with other colony members being socially subordinate and reproductively suppressed. To date, competition for breeding status has been examined in-colony, with female, but not male, aggression observed following the death/removal of established queens. To determine whether this sex difference extends to colony-founding contexts, and clarify neural and endocrine mechanisms underlying behavioral change in females competing for status, we examined neurogenesis and steroid hormone concentrations in colony-housed subordinates, and NMRs given the opportunity to transition status via pair-housing. To this end, Ki-67 and doublecortin immunoreactivity were compared in the hippocampal dentate gyrus (DG) and basolateral amygdala (BLA) of colony-housed subordinates, and subordinates housed with a same-sex (SS) or opposite-sex (OS) conspecific. Results suggest that OS pairing in eusocial mammals promotes cooperation and enhances hippocampal plasticity, while SS pairing is stressful, resulting in enhanced HPA activation and muted hippocampal neurogenesis relative to OS pairs. Data further indicate that competition for status is confined to females, with female-female housing exerting contrasting effects on hippocampal and amygdalar neurogenesis. These findings advance understanding of social stress effects on neuroplasticity and behavior, and highlight the importance of including female-dominated species in research on aggression and intrasexual competition.